A high alloy chromium cast iron having suitable wear resistance and cracking resistance and comprising 2 to 3.2% of C, 12 to 18% of Cr, 2% or less of Ni and 2% or less of Mo or a high alloy grain cast iron comprising 3 to 3.4% of C, 0.4 to 1.5% of Cr, 2 to 5% of Ni and 0.2 to 1.0% of Mo has hitherto been used as a roll for hot rolling (see "Tekko Zairyo no Mamo (Wear of Steel Material)", Joint Society on Iron and Steel Basic Research, page 16 (1984)). Further, Japanese Unexamined Patent Publication (Kokai) No. 58-87249 has proposed a wear-resistant cast iron roll material comprising 2.4 to 3.5% of C, 6.1 to 14% of V and alloying elements, that are Cr, Mo, W and Co, for improving the wear resistance.
With respect to a technique for producing this type of roll, as disclosed in, for example, Japanese Examined Patent Publication (Kokoku) No. 59-19786 and Japanese Unexamined Patent Publication (Kokai) No. 61-60256, a technique is known wherein a cooling die is provided around a core material preheated by means of a preheating coil, introducing a molten metal into the cooling die while heating the molten metal by means of a heating coil to form an outer layer welded to around the core material.
Wear resistance and crack resistance are important to properties required of the rolling roll of the type described above. Smaller wear contributes to an improvement in the accuracy of sheet thickness of the rolled product and, at the same time, reduces the frequency of replacement of rolls, which in turn contributes to an improvement in the working efficiency.
When the crack resistance is insufficient, cracking occurs due to the application of thermal or mechanical load during use, which gives rise to a major problem trouble. Therefore, it is strongly desired for the rolling roll to have both the wear resistance and the crack resistance.
In this connection, as compared with the conventional high chromium cast iron and high alloy grain cast iron, a wear-resistant material proposed in Japanese Unexamined Patent Publication (Kokai) No. 58-87249 has been confirmed to have better wear resistance. In this material as well, it is necessary to further improve the crack resistance.
Surface roughening resistance and roll surface roughness are other properties required of the rolling roll. Specifically, the smaller the surface roughness of the roll applied to the rolling, the better the appearance of a product formed by rolling through the use of such a roll. For this reason, an improvement in the surface roughening resistance through a reduction in the surface roughness of the roll has been strongly desired in the art.